Photonic integrated circuits include multiple optical components integrated on a common substrate, typically a semiconductor substrate. The optical components may include arrays of elements such as waveguides, splitters, couplers, interferometers, modulators, filters, etc., and may have similar or different optical processing functions. Photonic integrated circuits may be built by bonding together several optical, electro-optical, or optoelectronic chips. Electrical driver chips may also be attached to optoelectronic chips and electrically coupled by solder bumps or wirebonds.
Structurally, photonic integrated circuits resemble electronic integrated circuits, with optical waveguides for conducting optical signals between different optical components. Due to integrated character of optical components and connections, photonic integrated circuits may be suitable for mass production to a similar degree integrated electronic circuits are, potentially allowing significant economy of scale. Silicon-based photonic integrated circuits in particular may benefit from a well-developed material, technological, and knowledge base of silicon-based microelectronics industry.
It may be desirable to reduce size of photonic integrated circuits to fit more circuits on a same semiconductor wafer. To achieve size reduction, individual circuit components need to be more densely packed. There is, however, a limit on how densely the components may be packed. When distances between the components are too small, optical crosstalk may result. The optical crosstalk occurs because light scattered from one component may be coupled to a nearby component, impacting that component's optical performance. Amplifiers, lasers, and photodetectors may be particularly sensitive to optical crosstalk caused by stray light from neighboring components.
One typical example of a light-scattering component is a Mach-Zehnder interferometer of an optical modulator. When light modes in two arms of the Mach-Zehnder interferometer are in counter phase, a Y-junction combiner combining the two arms does not couple light into the output waveguide of the Y-junction combiner. Instead, the light is coupled into a radiative mode, causing the light to scatter throughout the photonic integrated circuit. Another typical example of a light-scattering component is an in-coupler of light. An in-coupler disposed near an edge of a photonic integrated circuit may scatter light escaped the core of an input waveguide due to an optical misalignment, imperfection of the input optical mode, etc. The scattered light may become guided by various layers of the photonic integrated circuit, causing extensive “ringing”, i.e. optical crosstalk.
Thus, not only is optical crosstalk a limiting factor of miniaturization of photonic integrated circuits, it may also be a performance-degrading factor, and a significant design constraint. In prior-art photonic integrated circuits, the optical components are spaced apart to reduce the effect of optical crosstalk. This increases the overall dimensions of photonic integrated circuits, raising manufacturing costs.